1. Field of the Invention
This invention relates generally to environmental enclosures, and more particularly to an improved environmental enclosure structure formed of a cement-based slurry infiltrated fiber composite material wherein the walls contain a mass of short fibers or fiber mats of organic or inorganic materials having a predetermined fiber volume density completely infiltrated in a cement-based matrix mixture.
2. Brief Description of the Prior Art
Enclosure structures such as: secondary containment vaults for hazardous materials; underground storage vaults; controlled environment vaults for housing communication equipment ( telephone, computer, surveillance, etc. ); and high security vaults for storing volatile explosives, nuclear weapons, test devices, and weapons components, are usually fabricated using conventional reinforced and pre-stressed concrete.
To meet the structural design requirements for resisting hydrostatic loads, soil pressures, explosions, or concussion, the walls of the vaults are generally quite thick. Some of these structures are monolithic which requires that they be fabricated on the installation site, since the thickness of the walls and conventional steel reinforced concrete construction produces a structure which is too heavy to be transported or shipped as a single unit. As a result, many of these types of structures are manufactured in panels or sections and then transported and assembled at the installation site. To develop the required structural capacity of the vault wall, and to insure a leak-free joint between the sections, cables are sometimes used to draw the sections together after the components have been assembled and rubber gaskets and caulking are employed to make the joints leak free.
In the past, materials such as petroleum products, chemicals, and hazardous materials have been stored in large metal or fiberglass tanks which are buried underground. Most of these "underground fuel storage tanks" (UFST) are prone to leakage due to being subjected to the hydrostatic forces of ground water, physical stresses associated with ground movement, and the corrosive action of soil environments. These steel tanks are known to begin failing leakage tests or to begin leaking at a much greater frequency after about twelve years in operation. Great damage to the environment and personal injury often results when the leaked materials enter the soil or ground water.
The United States Environmental Protection Agency (EPA) has recently adopted new regulations for Underground Fuel Storage Tanks (UFST) in response to the growing awareness of the damage caused by releases from the UFST's. These regulations will require UFST owners to spend significant sums of money over the life of the storage tanks for monitoring, reporting, and corrective actions. Failure to comply with the EPA regulations could result in having to take the storage tank out of service, and the possibility of financial liability for property damage and personal injuries. The EPA has estimated that more than $69 billion will be spent over the next 30 years on UFST systems in leak detection, inspections, upgrading, and corrective actions.
One method to comply with the EPA regulations is to place the fuel storage tank inside a buried "secondary containment vault". The secondary containment vault is a box-like structure having an interior volume greater than the capacity of the tank it contains. Such a system provides the ability to easily monitor the tank for leakage. Should a leak occur, the secondary containment vault will completely contain the leak, preventing the fuel from entering the soil or ground water. The secondary containment vault also isolates the fuel tank from soil and hydrostatic pressures and the corrosive action of many soils. Fuel tanks which are situated in secondary containment vaults in a manner to allow physical inspection are specifically excluded from EPA and most state regulations.
Most underground secondary containment vaults currently available are fabricated using conventional reinforced and pre-stressed concrete. To meet the structural design requirements for resisting hydrostatic and soil pressures, the walls of the vaults are generally from 8 to 10 inches thick. This produces a structure which is too heavy to be transported or shipped as a single unit. As a result, most conventional secondary containment vaults are manufactured in three parts; a monolithic lower section, an upper section, and a roof slab for the upper section. The roof slab is manufactured in several panels. To develop the required structural capacity of the vault wall, and to insure a leak-free joint between the lower and upper sections, post tensioned cables are used to draw the two sections together after the components have been assembled in the excavation. Rubber gaskets and caulking are employed to make the joint leak free. Such a secondary containment vault is manufactured by Unisil of Reston, Va.
Controlled environment vaults are box-like structures which are used for housing communication equipment, such as telephone, computer, or surveillance equipment, etc., which requires a controlled environment for proper operation. The controlled environment vaults may contain temperature control equipment, dehumidifiers, fresh air blowers, environment monitors and alarms, and electrical control panels and outlets, etc. The controlled environment vaults may be partially buried with an entry hatch above ground. Controlled environment vaults range in size from about 17'-25' in length, 7'-12' in height, and 10'-12' in width. A controlled environment vault of conventional steel reinforced concrete in the smaller size has a weight of 70,000 lbs, and the larger size weighs about 140,000 lbs, with a concrete strength of 5,000 psi.
High security vaults constructed of conventional reinforced concrete are used for storing volatile explosives, nuclear weapons, test devices, and weapons components, where high strength and security is a factor.
Utility Vault Company, Inc., of Chandler, Ariz. manufactures secondary containment vaults, and controlled environment vaults which are constructed of conventional steel reinforced concrete.
There are several patents which disclose various fiber reinforced concrete structures.
U.S. Pat. No. 3,429,094 to Romualdi discloses a two-phase concrete and steel material comprising closely spaced short wire segments uniformly distributed randomly in concrete wherein the average spacing between wire segments is not greater than 0.5 inches.
Fleischer et al, U.S. Pat. No. 4,257,912 discloses a system for fixed storage of spent nuclear fuel having activated fission products contained within a metallic fuel rod housing which comprises a uniform concrete contiguously and completely surrounding the metallic housing which has metallic fibers to enhance thermal conductivity and polymers to enhance impermeability for convectively cooling the exterior surface of the concrete.
Lankard et al, U.S. Pat. No. 4,559,881 discloses a burglar resistant security vault formed of prefabricated steel fiber reinforced concrete modular panels wherein Portland Cement, fly ash, fine aggregate, gravel and water are mixed for an extraordinarily long period of time and they remain a mass of crumbly, damp, powder and aggregate until the superplasticizer admixture is added, at which time the mixture reaches a fluid state. Steel fibers are then added to the mixture and mixing continues until the mixture including the steel fibers is poured into a mold cavity.
Double et al, U.S. Pat. No. 4,780,141 discloses a cementious composite material containing metal fiber which particularly formulated to have high strength and a high degree of vacuum integrity at high temperatures. The composite comprises a high strength cement matrix and a filler component comprising a metal fiber having a length of about 0.05 mm. to about 5 mm. (about 0.02" to about 0.20"). The metal fiber filler is mixed with the cement matrix at a high vacuum to minimize air bubbles and then the liquid mixture (including metal fiber) is poured into the mold.
Heintzelman et al, U.S. Pat. No. 5,030,033 discloses a conventional concrete underground .storage vault comprised of a plurality of concrete sections sealingly secured together with grout keys and joint wrap. A fluid and material resistant (epoxy) coating is applied to the interior surfaces and an inert gas atmosphere is maintained within the vault to inhibit influx of oxygen and moisture. There is no teaching in Heintzelman of the type of concrete used, other than "precast concrete" or "steel and/or concrete".
Riley et al, U.S. Pat. No. 4,133,928 discloses a composite cementious or gypsum matrix material having precombined absorbent fibres and reinforcing fibre embedded therein. The absorbent fibres are selected from the group consisting of cotton, wool, cellulose, viscose rayon, and cuprammonium rayon, with the reinforcing fibers being selected from the group consisting of glass, steel, carbon, polyethylene and polypropylene. The fibre combinations are impregnated with portland cement or gypsum. Riley et al teaches a steel wire/cotton yarn reinforced concrete made by loom weaving a tape or felt having ten ends per inch for each fibre in both the longitudinal (warp) and cross (weft) directions then passing the tapes through a portland cement mortar slurry consisting of one part water, two parts cement, three parts sand by weight, and then winding the tapes into a mold and placing the mold in a curing room for one month.
As described hereinafter, the present invention utilizes a "cement-based slurry infiltrated fiber composite" construction which is significantly different from conventional "steel bar reinforced concrete" "steel fiber reinforced concrete" and "pre-stressed concrete", in both its fiber volume density and in the manner in which it is made. The "cement-based slurry infiltrated fiber composite" described hereinafter overcomes the disadvantages of conventional concrete constructions and produces a structure which has thinner walls and a gross weight significantly less than conventional reinforced and pre-stressed concrete structures of the same size and has the same or greater strength characteristics, and a much higher bending capacity approximating that of structural steel.
The present invention is distinguished over the prior art in general, and these patents in particular by an environmental enclosure structure formed of a cement-based slurry infiltrated fiber composite material which is used above ground or underground to enclose, protect, and safely contain; hazardous materials, telecommunications equipment, volatile explosives, and the like. The preferred enclosure structure is a box-like enclosure formed of a cement-based slurry infiltrated fiber composite material which is produced by first placing a plurality of individual short fibers or fiber mats of organic or inorganic materials into a form to create a bed of fibers substantially filling the form and having a predetermined fiber volume density and then adding a cement-based slurry mixture into the form to completely infiltrate the spaces between the fibers. The cement-based slurry mixture includes a composition of Portland cement, fly ash, water, a high-range water reducer (superplasticizer), and may also include fine grain sand, chemical admixtures, and other additives. Due to its fiber volume density and method of manufacture, the resulting structure has thinner walls, greater strength, and a gross weight significantly less than conventional reinforced and pre-stressed concrete structures of the same size, and a much higher bending capacity approximating that of structural steel.